Xenon-Ion propulsion systems have become widely used on Geosynchronous Orbit (GEO) communications spacecraft. These systems have much higher efficiency than chemical systems and can be an effective means to increase the payload mass delivered to orbit or enable launch on a less expensive and less capable launch vehicle. However, there are several significant issues affecting Xenon-Ion thruster use. First, their high energy plumes can erode spacecraft surfaces and distort communications signals. Also, they are expensive, heavy, and require high power to operate. Therefore, they are of greatest utility on large, high power communications spacecraft with high payload mass. Also, they are of the greatest benefit when used for partial orbit transfer, in addition to mission orbit stationkeeping. These issues constrain the ion-thruster mounting and gimbaling arrangements, which in turn affect how they must be operated and their overall efficiency.
Two standard ion-thruster configurations and orbit control methods are presently in use. One approach involves the use of four ion thrusters located on the aft end (anti-earth panel) of the spacecraft. Each thruster generates in-track, cross-track, and radial thrust components. The thrusters are placed on separate gimbaled platforms and positioned to allow both North/South (N/S) and East/West (E/W) stationkeeping. The drawback of this approach is reduced propellant efficiency and added operational complexity in the event of a single thruster failure. In fact, in this approach, if a thruster fails, three maneuvers must be executed to change the orbit inclination. Two maneuvers are used to change inclination, and a third maneuver corrects for the in-track perturbation effect of the first two. Because, propellant must be carried to ensure mission life for the failure case, spacecraft payload capability is reduced.
A second approach uses two articulated booms with two ion thrusters mounted on each boom. The booms are mounted on the north and south panels towards the aft end of the spacecraft. With this arrangement each thruster can only generate cross-track and radial thrust components. The advantage of this arrangement is that the booms can be oriented for orbit raising or N/S stationkeeping. Also, performance is unaffected by the failure of any one of the thrusters. The disadvantage of this arrangement is that it cannot support E/W stationkeeping. Therefore, additional thrusters or a separate propulsion system must be provided for this purpose, with added cost, complexity, and mass.
Accordingly, an improved ion-thruster orbit control method and configuration is desired that provides increased propellant efficiency, and does not require the use of a separate propulsion system for E/W stationkeeping. Also, an orbit control method is desired that results in only a nominal reduction in efficiency if a single thruster fails and that minimizes the required number of orbit control maneuvers.